This invention relates generally to a method of administering radiation therapy and more particularly to a method of minimizing the distortion of the radiation isodose contours created by tissue surface irregularities and of targeting the depth of maximum dose at an effective tissue depth during radiation therapy to a treatment field from a radiation source.
Magnetic resonance imaging and radiation therapy are well known and effective treatment and diagnostic methods for various types of disease. During the treatment of skin cancers and various other conditions, it is sometimes therapeutically beneficial to expose surface and near-surface tissues of the human body to predetermined doses of radiation. Although beneficial, radiation therapy of surface and near-surface tissues is difficult to perform.
One of the principal difficulties associated with surface and near-surface radiation therapy when using radiation in the mega-electron-volt range is applying the maximum dose applied to the treatment field. The treatment field is that tissue, such as a tumor, for which application of the radiation is the prescribed treatment. Dosage control is difficult because radiation, in the form of an electrons, which strikes a relatively high density structure like human tissue has its highest therapeutic intensity level, not at the surface of the tissue, but at a depth below the surface. The depth at which the highest dose of radiation is received is known as the depth of maximum dose. Depth of maximum dose is a function of the energy imparted to the electrons by the source and the physical properties of the material at which the electrons are targeted.
When the electrons first enter tissue they strike atoms knocking electrons free from some of them. The electrons which have been knocked free are known as secondary electrons. The secondary electrons, in turn, knock other electrons free; creating geometric growth in the number of free electrons within the tissue, until a portion of the energy in the original electrons is attenuated. The tissue depth at which the chain reaction knocks free the largest number of electrons is the depth of maximum dose. This depth, depending on the energy level of the electrons emitted from the source, can range from a few millimeters to a few centimeters.
It is, therefore, the general practice, when administering radiation therapy to treatment fields comprising surface and near-surface tissue structures, to bolus the area above the site to receive the radiation. Bolusing an area means placing a material having radiological characteristics equivalent to tissue in contact with the tissue surface; between the tissue surface and the radiation source. The depth of maximum dose is then raised to the treatment field by selecting and applying an appropriate thickness of bolus material to the area above the treatment field tissue.
Bolusing materials are also used when radiation in the form of photons is used during radiation therapy. There are two purposes for bolusing when using this form of radiation. The first purpose is to adjust the depth of maximum dose to a desired level. The second purpose is to minimize the distortion of the radiation isodose contours due to tissue surface irregularities.
An isodose contour is a representation of dosage level information. Those tissue areas receiving the same radiation dosage level form an isodose contour.
When radiation enters tissue through an area of surface tissue which is normal to the direction of propagation of the wave, the isodose contours form a uniform gradient parallel to the plane of the surface tissue. This type of isodose contour is known as an homogeneous isodose contour.
When radiation enters tissue through an area of surface tissue which is not normal to the direction of propagation of the wave, the direction of propagation changes at the air to surface tissue interface. This change in direction causes interference patterns and creates isodose contours having various shapes. This type of isodose contour is known as a non-homogeneous isodose contour. As can be expected, those surface tissue areas having the greatest surface variations, such as the buttocks area, create greater non-homogeneity in the isodose contours.
It is generally known to use bolus material to increase the homogeneity of the isodose contours during radiation therapy. This is accomplished by using bolus material to increase the quantity of surface area normal to the direction of propagation of the radiation wave by either (i) placing a sheet of bolus material over the irregularly shaped surface tissue or by (ii) placing bolus material within the surface variations themselves. However, because of the great variety of surface variations which exist and the mechanical properties of conventional bolusing materials, air pockets are often formed between the bolusing material and the tissue surface.
The formation of air pockets is hard to avoid when using conventional bolusing materials during radiation therapy to irregularly shaped tissue structures. For instance, when it is necessary to apply radiation to a structure such as the ear, the ear is bolused and the radiation administered. Because of the ears physical form, the bolusing material may not conform entirely to the contours of the surface structure of the ear and air pockets may be formed. These air pockets can create isodose contour distortion; limiting the benefit realized by the use of the bolus. It is, therefore, desirable to have a method of bolusing prior to radiation therapy which will minimize air pocket formation.
The solution to minimizing air pocket formation is to preform the bolus into a shape which conforms to the shape of the surface tissue. This can be accomplished by (i) making a negative latex cast of the tissue structure, (ii) making a positive mold from the negative cast, and (iii) creating a bolus out of a material such as bees wax using the positive mold. This is a time consuming and expensive process. A bees wax bolus for an ear can take as long as eight hours to manufacture. A method of preforming boluses which is quick and inexpensive is therefore desirable.
During the course of radiation treatment, it necessary to perform magnetic resonance imaging of the target field and surrounding tissue structures to evaluate treatment progress and the degree of radiation damage to the surrounding tissue structures caused by the therapy. A treatment portal is the volume of non-treatment field tissue that the radiation must pass through in order to treat the treatment field tissue. The treatment portal begins at one skin surface, continues through the patient and terminates at the opposite skin surface of the patient. Magnetic resonance imaging is helpful in reducing the amount of non-treatment field tissue damage by providing the prescribing physician with information regarding the degree of damage done to non-treatment field tissue. It is important in evaluating and detecting damage to non-treatment field tissue areas that the physician be able to determine from the MRI the treatment portal through which the radiation treatments are directed. It is required during prolonged radiation treatments to change the treatment portal in order to minimize damage to tissue structures within a given treatment portal. A method for marking the entrance and exit portions of the portal on the patient prior to performing the MRI is therefore desirable in order to minimize the damage to non-treatment field tissue areas.